(1) Field of the Invention
The present invention relates to lubricating grease for high temperature application and to a rolling bearing. More particularly, the present invention relates to fluorine-containing lubricating grease, for high temperature application, that can be used suitably for electric auxiliaries used for cars and at a fixing portion of an electrophotographic apparatus; and a rolling bearing in which the fluorine-containing lubricating grease is used. The present invention also relates to a rolling bearing which is used in a boundary lubrication condition such as in vacuum where the grease is enclosed in the bearing in a comparatively small amount.
(2) Description of Related Art including information disclosed under 37 CFR1.97 and 1.98
The lubricating grease is enclosed in the rolling bearing to impart lubricating performance thereto. The lubricating grease is obtained by kneading a base oil and a thickener both serving as its main component. As the base oil, mineral oil and synthetic oil such as ester oil, silicone oil, and ether oil are used. As the thickener, metal soap such as lithium soap and urea compounds are conventionally used. The lubricating grease contains additives such as an antioxidant, a rust preventive agent, a metal deactivator, a viscosity index improver, and the like as necessary.
In recent years, there are growing demands for production of a car which is compact, lightweight, and has low degree of noise. Therefore efforts are made to produce automotive electric components which are compact, lightweight, and have low degree of noise and make the interior of the engine room airtight. The electric component parts are also required to have high output and efficiency. Rolling bearings are used at shafts of many electric auxiliary machines of cars. Because the life of the lubricating grease to be enclosed in the rolling bearing is shorter than the fatigue-caused life of the rolling bearing, the life of the rolling bearing depends on the life of the lubricating grease. Therefore the lubricating grease used for automotive electric components is demanded to be more resistant to heat than urea-containing lubricating grease.
Heretofore heat-resistant fluorine-containing lubricating grease (hereinafter referred to as fluorine-containing grease) containing fluorocarbon resin powder as its thickener and perfluoropolyether (hereinafter referred to as PFPE) oil as its base oil is used as grease to be enclosed in the rolling bearing for use in a fan clutch which is heated to high temperatures up to 200° C.
Many rolling bearings for a roll for rotatably supporting a fixing roller or the like are used at a fixing portion of a copying machine, a printing machine (printer), and the like in which an electrophotographic apparatus is mounted. At the fixing portion, toner which has been charged and attached to paper is fixed thereto by applying a pressure to the toner up to a temperature as high as 250° C. In many cases, the rolling bearing for rotatably supporting the roll at the fixing portion is used at a high temperature. Especially in the case of a heat roll, there is a case in which the bearing is used at a temperature higher than 200° C. because a heater is mounted inside a hollow shaft of a heat roll to heat it from the inside thereof. There is a case in which a rolling bearing for supporting the heat roll is used through a heat-insulating sleeve made of resin to decrease the temperature of the bearing portion thereof. Nevertheless there is a case in which the temperature of the end face of the bearing becomes nearly 200° C. by radiant heat. As grease enclosed in the rolling bearing used at such a high temperature, fluorine-containing grease which deteriorates to a low extent at a high temperature and has a long life is conventionally used.
When the lubricating grease is subjected to vacuum, base oil having a high vapor pressure evaporates. Therefore the vacuum degree does not rise or the lubricating grease pollutes measuring instruments provided inside a vacuum chamber and vacuum instruments, which leads to the occurrence of defects. To overcome the disadvantage, the fluorine-containing grease is frequently used to lubricate the rolling bearing used in vacuum.
The fluorine-containing grease displays preferable lubricating performance when a sufficiently large amount of grease is present. But when the amount of the supply of the fluorine-containing grease to a rolling contact portion and to a sliding portion is insufficient and boundary lubrication is performed, the PFPE oil serving as the base oil and steel (iron) serving as the material of the bearing react with each other. As a result, the base oil decomposes and the steel wears. Thus the bearing has a short life. This reaction is conspicuous in the sliding portion between a retainer and rolling elements. When the bearing is operated for a long time, the surfaces of the rolling elements are corroded and worn to a high extent. Owing to this reaction, the PFPE oil deteriorates and is consumed. Therefore the amount of the lubricant which can be utilized decreases conspicuously and owing to the multiplication of these disadvantages, there occurs a phenomenon that the rolling bearing has a short life and seizes up.
The rolling bearing is demanded to have a long life by restraining this reaction and utilizing the heat resistance and lubricating performance inherent in the PFPE oil. Known improvements to comply with the demand include the improvement of seizing resistance and wear resistance of the fluorine-containing grease by the formation of the silica film (see patent document 1); the improvement of the rolling fatigue characteristic by the mixing of the organic antimony or the organic molybdenum (see patent document 2); the improvement of the rolling fatigue characteristic by the mixing of the bismuth compound (see patent document 3); the improvement of the wear resistance and rust preventive performance of the fluorine-containing grease by the mixing of the fluorine organic phosphorous compound (see patent document 4); the improvement of the wettability of the fluorine-containing grease to metal by the mixing of the laminar mineral powder such as molybdenum disulfide, synthetic mica, and the like, metal salt such as metal soap, metal oxide, and the like, carbon compounds such as diamond powder, graphite, and the like, melamine cyanurate, and an amino acid compound (see patent document 5); and the prevention of the deterioration of the fluorine-containing grease by the mixing of the modified undecanoic mixture, the modified butane, Cu phthalocyanine, and Ca sulfonate (see patent document 6).
However, these improvements are not made in view of the reaction between fluorine and newly generated ferrous metal surface such as a surface of steel. Thus the properties of the fluorine-containing grease are not improved radically.
The rolling element of the conventional rolling bearing is made of steel. To allow the rolling element of the rolling bearing to be corrosion-resistant and heat-resistant in a corroding environment and a high-temperature environment, there is a case where ceramic is used partly for the rolling element. But the fluorine-containing grease is not used for the ceramic rolling element. When the fluorine-containing grease superior in durability at a high temperature is used, as a measure for preventing the rolling element from being corroded by a generated fluorine gas, a rolling bearing in which the rolling element is made of ceramics instead of steel has not been put into practical use.
Patent document 1: Japanese Patent Application Laid-Open No. 2005-97513
Patent document 2: Japanese Patent Application Laid-Open No. 2000-303088
Patent document 3: Japanese Patent Application Laid-Open No. 2005-42102
Patent document 4: Japanese Patent Application Laid-Open No. 2003-27079
Patent document 5: Japanese Patent Application Laid-Open No. 2004-188607
Patent document 6: Japanese Patent Application Laid-Open No. 8-143883